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1、Chapter 4:Cell Membrane and Cell Surface I.Cell Membrane II.Cell Junctions III.Cell Adhesion IV.Extracellular Matrix I.Biomembranes:Their Structure,Chemistry and Functions Learning objectives:1.A brief history of studies on the structrure of the plasma membrane2.Model of membrane structure:an experi
2、mental perspective3.The chemical composition of membranes4.Characteristics of biomembrane5.An overview of the functions of biomembranes 1.1.A brief history of studies on the structrure of the plasmic membraneA.Conception:Plasma membrane(cell membrane),Intracellular membrane,Biomembrane.B.The history
3、 of studyOverton(1890s):Lipid nature of PM;J.D.Robertson(1959):TheTEMshowing:thetrilaminarappearanceofPM;Unit membrane model;S.J.Singer and G.Nicolson(1972):fluid-mosaicmodel;K.Simonsetal(1997):lipidraftsmodel;FunctionalraftsinCellmembranes.Nature387:569-572 2.Singer and Nicolsons Model of membrane
4、structure:The fluid-mosaic model is the“central dogma”of membrane biology.A.The core lipid bilayer exists in a fluid state,capable of dynamic movement.B.Membrane proteins form a mosaic of particles penetrating the lipid to varying degrees.The Fluid Mosaic Model,proposed in 1972 by Singer and Nicolso
5、n,had two key features,both implied in its name.3.The chemical composition of membranesA.Membrane Lipids:The Fluid Part of the Model Phospholipids:Phosphoglycerideandsphingolipids Glycolipids Sterols(isonlyfoundinanimals)vMembrane lipids are amphipathic.vThere are three major classes of lipids:Figur
6、e 10-2.The parts of a phospholipid molecule.Phosphatidylcholine,representedschematically(A),informula(B),asaspace-fillingmodel(C),andasasymbol(D).Thekinkduetothecis-doublebondisexaggeratedinthesedrawingsforemphasis.Figure 10-3.A lipid micelle and a lipid bilayer seen in cross-section.Lipidmoleculesf
7、ormsuchstructuresspontaneouslyinwater.Theshapeofthelipidmoleculedetermineswhichofthesestructuresisformed.Wedge-shapedlipidmolecules(above)formmicelles,whereascylinder-shapedphospholipidmolecules(below)formbilayers.Figure 10-4.Liposomes.(A)Anelectronmicrographofunfixed,unstainedphospholipidvesicles(l
8、iposomes)inwater.Thebilayerstructureofthevesiclesisreadilyapparent.(B)Adrawingofasmallsphericalliposomeseenincross-section.Liposomesarecommonlyusedasmodelmembranesinexperimentalstudies.(A,courtesyofJeanLepault.)Figure 10-5.A cross-sectional view of a synthetic lipid bilayer,called a black membrane.T
9、hisplanarbilayerisformedacrossasmallholeinapartitionseparatingtwoaqueouscompartments.Blackmembranesareusedtomeasurethepermeabilitypropertiesofsyntheticmembranes.Figure 10-6.Phospholipid mobility.Thetypesofmovementpossibleforphospholipidmoleculesinalipidbilayer.Figure 10-7.Influence of cis-double bon
10、ds in hydrocarbon chains.Thedoublebondsmakeitmoredifficulttopackthechainstogetherandthereforemakethelipidbilayermoredifficulttofreeze.Figure 10-8.The structure of cholesterol.Cholesterolisrepresentedbyaformulain(A),byaschematicdrawingin(B),andasaspace-fillingmodelin(C).Figure 10-9.Cholesterol in a l
11、ipid bilayer.Schematicdrawingofacholesterolmoleculeinteractingwithtwophospholipidmoleculesinoneleafletofalipidbilayer.Figure 10-10.Four major phospholipids in mammalian plasma membranes.Notethatdifferentheadgroupsarerepresentedbydifferentsymbolsinthisfigureandthenext.Allofthelipidmoleculesshownarede
12、rivedfromglycerolexceptforsphingomyelin,whichisderivedfromserine.Figure 10-11.The asymmetrical distribution of phospholipids and glycolipids in the lipid bilayer of human red blood cells.ThesymbolsusedforthephospholipidsarethoseintroducedinFigure10-10.Inaddition,glycolipidsaredrawnwithhexagonalpolar
13、headgroups(blue).Cholesterol(notshown)isthoughttobedistributedaboutequallyinbothmonolayers.Figure 10-12.Glycolipid molecules.Galactocerebroside(A)iscalledaneutral glycolipidbecausethesugarthatformsitsheadgroupisuncharged.Aganglioside(B)alwayscontainsoneormorenegativelychargedsialicacidresidues(alsoc
14、alledN-acetylneuraminicacid,orNANA),whosestructureisshownin(C).Whereasinbacteriaandplantsalmostallglycolipidsarederivedfromglycerol,asaremostphospholipids,inanimalcellstheyarealmostalwaysproducedfromsphingosine,anaminoalcoholderivedfromserine,asisthecaseforthephospholipidsphingomyelin.Gal=galactose;
15、Glc=glucose,GalNAc=N-acetylgalactos-amine;thesethreesugarsareuncharged.Figure 10-13.Six ways in which membrane proteins associate with the lipid bilayer.Mosttrans-membraneproteinsarethoughttoextendacrossthebilayerasasingleahelix(1)orasmultipleahelices(2);someofthesesingle-passandmultipassproteinshav
16、eacovalentlyattachedfattyacidchaininsertedinthecytoplasmicmonolayer(1).Othermembraneproteinsareattachedtothebilayersolelybyacovalentlyattachedlipid-eitherafattyacidchainorprenylgroup-inthecytoplasmicmonolayer(3)or,lessoften,viaanoligosaccharide,toaminorphospholipid,phosphatidylinositol,inthenoncytop
17、lasmicmonolayer(4).Finally,manyproteinsareattachedtothemembraneonlybynoncovalentinteractionswithothermembraneproteins(5)and(6).Howthestructurein(3)isformedisillustratedinFigure10-14.Membrane proteinsFigure 10-14.The covalent attachment of either of two types of lipid groups can help localize a water
18、-soluble protein to a membrane after its synthesis in the cytosol.(A)Afattyacidchain(eithermyristicorpalmiticacid)isattachedviaanamidelinkagetoanamino-terminalglycine.(B)Aprenylgroup(eitherfarnesyloralongergeranylgeranylgroup-bothrelatedtocholesterol)isattachedviaathioetherlinkagetoacysteineresiduet
19、hatisfourresiduesfromthecarboxylterminus.Followingthisprenylation,theterminalthreeaminoacidsarecleavedoffandthenewcarboxylterminusismethylatedbeforeinsertionintothemembrane.Thestructuresoftwolipidanchorsareshownunderneath:(C)amyristylanchor(a14-carbonsaturatedfattyacidchain),and(D)afarnesylanchor(a1
20、5-carbonunsaturatedhydrocarbonchain).Figure 10-15.A segment of a transmembrane polypeptide chain crossing the lipid bilayer as an a helix.Onlythea-carbonbackboneofthepolypeptidechainisshown,withthehydrophobicaminoacidsingreen andyellow.(J.Deisenhoferetal.,Nature318:618-624andH.Micheletal.,EMBO J.5:1
21、149-1158)Figure 10-17.A typical single-pass transmembrane protein.Notethatthepolypeptidechaintraversesthelipidbilayerasaright-handedahelixandthattheoligosaccharidechainsanddisulfidebondsareallonthenoncytosolicsurfaceofthemembrane.Disulfidebondsdonotformbetweenthesulfhydrylgroupsinthecytoplasmicdomai
22、noftheproteinbecausethereducingenvironmentinthecytosolmaintainsthesegroupsintheirreduced(-SH)form.Figure 10-18.A detergent micelle in water,shown in cross-section.Becausetheyhavebothpolarandnonpolarends,detergentmoleculesareamphipathic.Figure 10-19.Solubilizing membrane proteins with a mild detergen
23、t.Thedetergentdisruptsthelipidbilayerandbringstheproteinsintosolutionasprotein-lipid-detergentcomplexes.Thephospholipidsinthemembranearealsosolubilizedbythedetergent.Figure 10-20.The structures of two commonly used detergents.Sodiumdodecylsulfate(SDS)isananionicdetergent,andTritonX-100isanonionicdet
24、ergent.Thehydrophobicportionofeachdetergentisshowningreen,andthehydrophilicportionisshowninblue.NotethatthebracketedportionofTritonX-100isrepeatedabouteighttimes.Figure 10-21.The use of mild detergents for solubilizing,purifying,and reconstituting functional membrane protein systems.Inthisexamplefun
25、ctionalNa+-K+ATPasemoleculesarepurifiedandincorporatedintophospholipidvesicles.TheNa+-K+ATPaseisanionpumpthatispresentintheplasmamembraneofmostanimalcells;itusestheenergyofATPhydrolysistopumpNa+outofthecellandK+in,asdiscussedinChapter11.Figure 10-22.A scanning electron micrograph of human red blood
26、cells.Thecellshaveabiconcaveshapeandlacknuclei.(CourtesyofBernadetteChailley.)Figure 10-24.SDS polyacrylamide-gel electrophoresis pattern of the proteins in the human red blood cell membrane.Thegelin(A)isstainedwithCoomassieblue.Thepositionsofsomeofthemajorproteinsinthegelareindicatedinthedrawingin(
27、B);glycophorinisshowninred todistinguishitfromband3.Otherbandsinthegelareomittedfromthedrawing.Thelargeamountofcarbohydrateinglycophorinmoleculesslowstheirmigrationsothattheyrunalmostasslowlyasthemuchlargerband3molecules.(A,courtesyofTedSteck.)Figure 10-25.Spectrin molecules from human red blood cel
28、ls.Theproteinisshownschematicallyin(A)andinelectronmicrographsin(B).Eachspectrinheterodimerconsistsoftwoantiparallel,looselyintertwined,flexiblepolypeptidechainscalledaandbtheseareattachednoncovalentlytoeachotheratmultiplepoints,includingbothends.Thephosphorylatedheadend,wheretwodimersassociatetofor
29、matetramer,isontheleft.Boththeaandbchainsarecomposedlargelyofrepeatingdomains106aminoacidslong.In(B)thespectrinmoleculeshavebeenshadowedwithplatinum.(D.W.SpeicherandV.T.Marchesi,Nature311:177-180;B,D.M.Shottonetal.,J.Mol.Biol.131:303-329)Figure 10-26.The spectrin-based cytoskeleton on the cytoplasmi
30、c side of the human red blood cell membrane.Thestructureisshownschematicallyin(A)andinanelectronmicrographin(B).Thearrangementshownin(A)hasbeendeducedmainlyfromstudiesontheinteractionsofpurifiedproteinsin vitro.Spectrindimersassociatehead-to-headtoformtetramersthatarelinkedtogetherintoanetlikemeshwo
31、rkbyjunctionalcomplexescomposedofshortactinfilaments(containing13actinmonomers),tropomyosin,whichprobablydeterminesthelengthoftheactinfilaments,band4.1,andadducin.Thecytoskeletonislinkedtothemembranebytheindirectbindingofspectrintetramerstosomeband3proteinsviaankyrinmolecules,aswellasbythebindingofb
32、and4.1proteinstobothband3andglycophorin(notshown).Theelectronmicrographin(B)showsthecytoskeletononthecytoplasmicsideofaredbloodcellmembraneafterfixationandnegativestaining.(B,courtesyofT.ByersandD.Branton,PNSA.82:6153-6157)Figure 10-31.The three-dimensional structure of a bacteriorhodopsin molecule.
33、Thepolypeptidechaincrossesthelipidbilayerassevenahelices.Thelocationofthechromophoreandtheprobablepathwaytakenbyprotonsduringthelight-activatedpumpingcycleareshown.Whenactivatedbyaphoton,thechromophoreisthoughttopassanH+tothesidechainofasparticacid85.Subsequently,threeotherH+transfersarethoughttocom
34、pletethecyclefromasparticacid85totheextra-cellularspace,fromasparticacid96tothechromophore,andfromthecytosoltoasparticacid96.(R.Hendersonetal.J.Mol.Biol.213:899-929)Figure 10-32.The three-dimensional structure of a porin trimer of Rhodobacter capsulatus determined by x-ray crystallography.(A)Eachmon
35、omerconsistsofa16-strandedantiparallelbbarrelthatformsatransmembranewater-filledchannel.(B)Themonomerstightlyassociatetoformtrimers,whichhavethreeseparatechannelsforthediffusionofsmallsolutesthroughthebacterialoutermembrane.Alongloopofpolypeptidechain(showninred),whichconnectstwobstrands,protrudesin
36、tothelumenofeachchannel,narrowingittoacross-sectionof0.6x1nm.(AdaptedfromM.S.Weissetal.,FEBS Lett.280:379-382)Figure 10-33.The three-dimensional structure of the photosynthetic reaction center of the bacterium Rhodopseudomonas viridis.Thestructurewasdeterminedbyx-raydiffractionanalysisofcrystalsofth
37、istransmembraneproteincomplex.Thecomplexconsistsoffoursubunits,L,M,H,andacytochrome.TheLandMsubunitsformthecoreofthereactioncenter,andeachcontainsfiveahelicesthatspanthelipidbilayer.Thelocationsofthevariouselectroncarriercoenzymesareshowninblack.(AdaptedfromadrawingbyJ.RichardsonbasedondatafromJ.Dei
38、senhoferetal.,Nature318:618-624)4.Characteristics of biomembraneA.Dynamic nature of biomembranevFluidity of membrane lipid.It give membranes the ability to fuse,form networks,and separate charge;vMotility of membrane protein.The lateral diffusion of membrane lipids can demonstrated experimentally by
39、 a technique called Fluorescence Recovery After Photobleaching(FRAP).Figure 10-34.Experiment demonstrating the mixing of plasma membrane proteins on mouse-human hybrid cells.Themouseandhumanproteinsareinitiallyconfinedtotheirownhalvesofthenewlyformedheterocaryonplasmamembrane,buttheyintermixwithtime
40、.Thetwoantibodiesusedtovisualizetheproteinscanbedistinguishedinafluorescencemicroscopebecausefluoresceinisgreenwhereasrhodamineisred.(BasedonobservationsofL.D.FryeandM.Edidin,J.Cell Sci.7:319-335)Figure 10-35.Antibody-induced patching and capping of a cell-surface protein on a white blood cell.Thebi
41、valentantibodiescross-linktheproteinmoleculestowhichtheybind.Thiscausesthemtoclusterintolargepatches,whichareactivelyswepttothetailendofthecelltoformacap.Thecentrosome,whichgovernsthehead-tailpolarityofthecell,isshowninorange.Figure 10-37.Diagram of an epithelial cell showing how a plasma membrane p
42、rotein is restricted to a particular domain of the membrane.ProteinA(intheapicalmembrane)andproteinB(inthebasalandlateralmembranes)candiffuselaterallyintheirowndomainsbutarepreventedfromenteringtheotherdomain,atleastpartlybythespecializedcelljunctioncalledatight junction.Lipidmoleculesintheouter(non
43、cytoplasmic)monolayeroftheplasmamembranearelikewiseunabletodiffusebetweenthetwodomains;lipidsintheinner(cytoplasmic)monolayer,however,areabletodoso.Figure 10-38.Three domains in the plasma membrane of guinea pig sperm defined with monoclonal antibodies.Aguineapigspermisshownschematicallyin(A),whilee
44、achofthethreepairsofmicrographsshownin(B),(C),and(D)showscell-surfaceimmunofluorescencestainingwithadifferentmonoclonalantibody(ontheright)nexttoaphase-contrastmicrograph(ontheleft)ofthesamecell.Theantibodyshownin(B)labelsonlytheanteriorhead,thatin(C)onlytheposteriorhead,whereasthatin(D)labelsonlyth
45、etail.(CourtesyofSelenaCarrollandDianaMyles.)Figure 10-39.Four ways in which the lateral mobility of specific plasma membrane proteins can be restricted.Theproteinscanself-assembleintolargeaggregates(suchasbacteriorhodopsininthepurplemembraneofHalobacterium)(A);theycanbetetheredbyinteractionswithass
46、embliesofmacromoleculesoutside(B)orinside(C)thecell;ortheycaninteractwithproteinsonthesurfaceofanothercell(D).Figure 10-41.Simplified diagram of the cell coat(glycocalyx).Thecellcoatismadeupoftheoligosaccharidesidechainsofglycolipidsandintegralmembraneglycoproteinsandthepolysaccharidechainsonintegra
47、lmembraneproteoglycans.Inaddition,adsorbedglycoproteinsandadsorbedproteoglycans(notshown)contributetotheglycocalyxinmanycells.Notethatallofthecarbohydrateisonthenoncytoplasmicsurfaceofthemembrane.cell coatFigure 10-42.The protein-carbohydrate interaction that initiates the transient adhesion of neut
48、rophils to endothelial cells at sites of inflammation.(A)ThelectindomainofP-selectinbindstothespecificoligosaccharideshownin(B),whichispresentonbothcell-surfaceglycoproteinandglycolipidmolecules.Thelectindomainoftheselectinsishomologoustolectindomainsfoundonmanyothercarbohydrate-bindingproteinsinani
49、mals;becausethebindingtotheirspecificsugarligandrequiresextracellularCa2+,theyarecalledC-type lectins.Athree-dimensionalstructureofoneoftheselectindomains,determinedbyx-raycrystallography,isshownin(C);itsboundsugariscoloredblue.Gal=galactose;GlcNAc=N-acetylglucosamine;Fuc=fucose;NANA=sialicacid.5.An
50、 Overview of membrane functions 1.Define the boundaries of the cell and its organelles.2.Serve as loci for specific functions.3.provide for and regulate transport processes.4.contain the receptors needed to detect external signals.5.provide mechanisms for cell-to-cell contact,communication and adhes